Descriptions of genetically unexpected immunoglobulins (latent allotypes) in several species suggest that immunoglobulin gene complexes contain more genetic information than is normally expressed, a possibility that should require a considerable revision in current concepts of the genetic control of Ig expression. Our studies have emphasized latent allotypes in the rabbit and have attempted to determine conditions for in vivo induction of latent allotypes. In conjunction with these studies an ELISA has been developed. Other studies attempt to determine the molecular biological basis for latent allotypes. Rabbit-mouse hybridoma cells have been used as a source of mRNA of the Ig chain. This is being purified to use in construction of DNA probes for determining the structure of the Ig genome in the rabbit.